Gas turbine systems provide shaft work by the expansion of hot pressurized gas streams produced directly or indirectly by the combustion of solid, liquid, or gaseous fuels. A gas turbine system utilizes one or more air compressors mechanically linked with one or more turboexpanders to provide shaft work for transportation, power generation, industrial processes, and many other well-known applications.
Gas turbine systems known in the art can be classified as directly-fired, indirectly-fired, or combination systems. In a directly-fired gas turbine system, compressed air is combusted with a fuel, typically a gas or light hydrocarbon liquid, and the hot pressurized combustion gases are expanded directly in the turboexpander. Additional work can be recovered from the hot turboexpander exhaust, for example by generating steam for expansion in a steam turbine. Directly-fired gas turbine systems are open systems in which the turboexpander working fluid (i.e. combustion products) is discharged to the atmosphere after appropriate heat recovery.
In an indirectly-fired gas turbine system, a working fluid (typically air) is compressed in the gas turbine compressor, heated by indirect heat exchange with a hot external gas stream (typically obtained by the combustion of a solid, liquid, or gaseous fuel) to yield hot pressurized air, and expanded in the turboexpander to generate shaft work. The turboexpander exhaust may be used to preheat the combustion air or may be introduced directly into the combustion step. Indirectly-fired gas turbines can be operated in a closed cycle wherein a recirculating gaseous working fluid is compressed, heated, expanded, and cooled using a totally integrated compressor and turboexpander.
Combined gas turbine systems are known in the art in which a directly-fired gas turbine is integrated with an indirectly-fired gas turbine. In such systems the indirectly-fired gas turbine may be a partially-closed system in which a major portion of the indirectly-fired turbine exhaust is cooled and recycled to the compressor.
The types of gas turbine systems summarized above are described in detail in standard textbooks such as The Mechanical Engineer's Handbook, Edited by M. Kurtz, John Wiley & Sons, Inc, 1986, Chapter 72, pp. 1984-2009 and Marks' Standard Handbook for Mechanical Engineers, edited by E. A. Avallone and T. Baumeister III, McGraw-Hill Book Co., New York, Ninth Edition (1987), pp. 9-118 to 9-123.
The separation of air into its components is accomplished by compressing air, pretreating the compressed air as necessary to remove certain contaminants, and separating the purified compressed air by known methods of cryogenic distillation, pressure swing adsorption, permeable polymeric membranes, or high temperature ceramic mixed conductor membranes. Power for compressing the air can be provided by electric motors, gas or steam turbines, or combinations of electric motors and gas or steam turbine drivers. Compressor driver selection is dictated by numerous design factors such as the type of air separation process, size of the process plant, location, electricity cost, fuel availability, and potential for integration of the air separation process with the compressor driver. In addition, the air separation system and compressor driver may be integrated with a process utilizing the product(s) of the air separation process.
A gas turbine is the preferred compressor driver in a number of air separation processes. One of these is the integrated gasification combined cycle (IGCC) process in which coal or other carbonaceous material is gasified with oxygen and the produced gas is cleaned to yield a low-sulfur fuel gas. This fuel gas is utilized in a direct-fired gas turbine which drives a generator to produce electric power with reduced environmental emissions. The oxygen is produced by cryogenic air separation wherein some or all of the compressed air feed may be provided by the gas turbine compressor, and the nitrogen-rich byproduct gas from the air separation system is compressed and introduced into the gas turbine combustor.
A general review of the current art in IGCC power generation systems is given by D. M. Todd in an article entitled "Clean Coal Technologies for Gas Turbines" presented at the GE Turbine State-of-the-Art Technology Seminar, July 1993, pp. 1-18. A review of various integration techniques and the impact thereof on GCC economics is given in a paper by A. D. Rao et al entitled "Integration of Texaco TQ Gasification with Elevated Pressure ASU" presented at the 13.sup.th EPRI Conference on Gasification Power Plants, San Francisco, Calif., Oct. 19-21, 1994. In a paper entitled "Improved IGCC Power Output and Economics Incorporating a Supplementary Gas Turbine" presented at the 13th EPRI Conference on Gasification Power Plants, San Francisco, Calif., Oct. 19-21, 1994, A. R. Smith et al review several modes of integration between the gas turbine and the air separation unit in an IGCC process.
The utilization of the nitrogen-rich byproduct stream by compression and injection into the combustor of an IGCC system is described in representative U.S. Pat. Nos. 4,250,704; 4,697,415; 5,081,845; 5,406,786; and 5,740,673. Another method of utilizing the nitrogen-rich waste stream in an integrated air separation/gas turbine system is described in U.S Pat. Nos. 3,731,495; 4,019,314; and 5,406,786 wherein this stream is optionally heated and introduced directly into the gas turbine expander without prior compression.
The use of an indirectly-fired gas turbine with a cryogenic air separation system is described in Great Britain Patent Specification 1 455 960. An air separation unit is integrated with a steam generation system in which a nitrogen-rich waste stream is heated by indirect heat exchange with hot compressed air from the air separation unit main air compressor, the heated nitrogen-rich stream is further heated indirectly in a fired heater, and the final hot nitrogen-rich stream is work expanded in a dedicated nitrogen expansion turbine. The work generated by this expansion turbine drives the air separation unit main air compressor. The nitrogen expansion turbine exhaust and the combustion gases from the fired heater are introduced separately into a fired steam generator to raise steam, a portion of which may be expanded in a steam turbine to drive the air separation unit main air compressor. Cooled nitrogen is withdrawn from the steam generator and may be used elsewhere if desired. Optionally, the combustion gases from the fired heater are expanded in a turbine which drives a compressor to provide combustion air to a separate fired heater which heats the nitrogen-rich stream prior to expansion. In another option, the nitrogen expansion turbine exhaust and the combustion gases from the fired heater are combined and introduced into the economizer and air preheater sections of the fired steam generator.
An indirectly-fired gas turbine IGCC system is described in U.S. Pat. No. 4,785,621 wherein the fuel gas from the gasifier is introduced into a directly-fired gas turbine which generates power. A separate indirectly-fired gas turbine system provides extracted air for the air separation system, the exhaust gas from the directly-fired gas turbine expander heats compressed air by indirect heat exchange, and the heated compressed air is expanded in the indirectly-fired gas turbine expander. Nitrogen-rich waste gas from the air separation system is mixed with the compressed air before the indirect heating and expansion steps. Exhaust from the indirectly-fired gas turbine expander is discharged to the atmosphere or used for supplemental heat recovery.
Gas turbines are the preferred drivers in processes to separate air at high temperatures in ceramic mixed conductor membrane systems. Directly-fired gas turbines for this application are described in U.S. Pat. Nos. 4,545,787; 5,516,359; 5,562,754; 5,565,017; and 5,657,624. A directly-fired gas turbine is utilized with a ceramic mixed conductor membrane air separation system which provides oxygen to a direct reduction iron recovery process as described in U.S. Pat. No. 5,643,354. Indirectly-fired gas turbines for this application are described in U.S. Pat. No. 5,035,727 and in an article entitled "Coproduction of Power, Steam, and Oxygen in Coal or Low Quality Fuel Combustion Systems" in Research Disclosure, March 1995, pp. 181-186.
The turboexpander of a directly-fired gas turbine generally requires more maintenance and has a lower onstream operating availability than the turboexpander of an indirectly-fired gas turbine. This occurs because the motive gas in the directly-fired gas turbine contains combustion products including water, carbon dioxide, sulfur compounds, and particulates, and these byproducts can cause fouling, erosion, and corrosion of the turboexpander internals thereby decreasing operating availability. The motive gas in an indirectly-fired gas turbine, on the other hand, contains no combustion products and the turboexpander therefore will operate with reduced fouling, erosion, and corrosion problems and will have a higher operating availability. This advantage of an indirectly-fired gas turbine is offset by the requirement for a high temperature nonrecuperative type of gas-gas heat exchanger to heat the motive gas before expansion, which is not required in a directly-fired gas turbine. The higher operating availability of the indirectly-fired gas turbine, however, may make it an attractive driver choice in certain applications.
The present invention discloses the use of an indirectly-fired gas turbine system to provide compressed air feed for an air separation system in applications where high driver availability is required. The invention utilizes the integration of an indirectly-fired gas turbine with the air separation system for the production of oxygen with efficient utilization of the nitrogen-rich byproduct.